Merge pull request #10 from jerryneedell/jerryn_negative

 fix DHT22 handling of negative temperatures

Author: dhalbert

adafruit_dht.py

@@ -148,18 +148,24 @@ def measure(self):
                 for byte_start in range(0, 80, 16):
                     buf.append(self._pulses_to_binary(pulses, byte_start, byte_start+16))
 
-                # humidity is 2 bytes
                 if self._dht11:
+                    # humidity is 1 byte
                     self._humidity = buf[0]
                 else:
+                    # humidity is 2 bytes
                     self._humidity = ((buf[0]<<8) | buf[1]) / 10
 
-                # temperature is 2 bytes
                 if self._dht11:
+                    # temperature is 1 byte
                     self._temperature = buf[2]
                 else:
-                    self._temperature = ((buf[2]<<8) | buf[3]) / 10
-
+                    # temperature is 2 bytes
+                    # MSB is sign, bits 0-14 are magnitude)
+                    raw_temperature = (((buf[2] & 0x7f)<<8) | buf[3]) / 10
+                    # set sign
+                    if buf[2] & 0x80:
+                        raw_temperature = -raw_temperature
+                    self._temperature = raw_temperature
                 # calc checksum
                 chk_sum = 0
                 for b in buf[0:4]:
@@ -170,7 +176,6 @@ def measure(self):
                     # check sum failed to validate
                     raise RuntimeError("Checksum did not validate. Try again.")
 
-
             else:
                 raise RuntimeError("A full buffer was not returned.  Try again.")